What do blue and black edges mean in edit mode?
Blue(Cyan, to be exact) means it's marked 'Sharp'.  If you are using the default blender theme that is.  This colour can be different in different Themes.

The reason they are blue are probably because the original mesh you extracted these faces from, had all its edges marked 'Sharp'.  3D Scanned objects are known to have interesting stuff tagged with their surface information.  And to blender, those stuffs tagged sometimes are identified as 'Sharp' or other markings.

Black lines, those aren't on your mesh, are probably glitches, or they belong to a very huge object in the background.  You can try to select them in the current edit mode showed in the picture.  If you can select them, they exist inside current editing-object.  Otherwise they belong to other objects in your scene.  Note that Lamp and other type of objects also use black lines to show themselves.

Those on your mesh, obviously are the wires that create faces and connects vertices.